(1) Field of the Invention
The invention is related in general to air-cooled heating, ventilation and air conditioning (HVAC) fan coil units.
(2) Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Air cooled direct expansion (ACDX) units are common in residential heating, ventilation and air conditioning (HVAC) applications and buildings of less than 650 square meters (7000 square feet). In the split system 100 illustrated in FIG. 1, the fan 102 and evaporator coil 104 of the air handling unit (AHU) 106 are typically located inside the building, e.g., in a mechanical closet, while the condensing unit (CU) 108 is located outside the air conditioned space.
The CU 108 houses a compressor 110, DX valve 112, CU fan 114 and cooling coil 116. The cooling coil 116 is typically located in fenestrations on three or four sides in a square or rectangular CU plan, depending on manufacturer, and can also be circular where the CU plan is circular. In package units (not shown) which are typically roof mounted, the AHU and CU are integrated into a single exterior unit wherein the supply and return ducts pass directly through the roof to the unit.
In operation, cold refrigerant is supplied to the evaporator coil 104, and the AHU fan 102 blows air across the evaporator coil 104, cooling the air that is circulated into the rooms via supply air duct 118 and return air duct 120. The warm refrigerant from the evaporator coil 104 is compressed at compressor 110, cooled in the cooling coil 116, expanded across DX valve 112 and supplied to the evaporator coil 104 to complete the cycle.
The cooling coil 116 is typically provided with extended surfaces such as fins, over which air is drawn by the CU fan 114 to dissipate the heat collected in the refrigerant during the cooling cycle, and the hot air is exhausted above the CU 108 by the centrally located, top-mounted fan 114. The unit 100 is typically thermostatically controlled whereby the unit 100 is cycled on when the temperature of the room air exceeds a set point, and cycled off when the temperature is below the set point. The rate of refrigerant cooling is largely a function of the temperature of the air being pulled across the coil 116, and the on cycle time depends in turn on how quickly the unit shut off set point is satisfied.
The outside CU 108 in the ACDX unit 100 is typically installed where cooling air is hotter than expected and/or cooling air flow is restricted based on the congested characteristics or orientation of the install and hot exhaust air can recirculate to the intake. Although manufacturers of ACDX units generally recommend 450 mm (18 in.) clearance around the unit to the nearest structure, these install guidelines are frequently not followed, in many cases due to geometric constraints at the installation location, and cooling air flow can be restricted or blocked from entering portions of the condenser coil.
Even where the spacing recommendations are followed there can be an issue with unit placement. For example, where the compressor unit is installed close to the building, the upward exhaust stream can impinge on an eave and be directed back down toward the CU 108, or there may be Coanda effects. Where there are multiple ACDX units installed, the issues become worse. Often these units sit so close together that air volume is limited, and the intake air temperature is higher than desired, and sometimes even hotter than the temperature for which the unit is designed to achieve boiler plate unit design output. In these situations, the CU fan 114 may provide an inadequate cooling air velocity for the required temperature drop (ΔT) across the coil 116 to properly cool, resulting in a drop in efficiency and an excessively long on cycle.
The HVAC industry is constantly seeking simple, effective and low-cost ways to improve the design and efficiency of HVAC ACDX units and their installations.